Anchorage device for pre-stress reinforcement members



1 April 26, 1960 F. L. CONVERSY 2,934,364

ANCHORAGE DEVICE FOR PRE-STRESS REINFORCEMENT MEMBERS Filed Jan. 23, 1957 3 Sheets-Sheet 1 \NVENTOR Fmucou s LCwvensy Y m MMwJ/h ATTORNEYS April 26, 1960 F. L. CONVERSY Filed Jan. 23, 1957 April 26, 1960 F, CQNVERSY 2,934,364

ANCHORAGE DEVICE FOR RE-STRESS REINFORCEMENT MEMBERS 3 Sheets-Sheet 3 Filed Jan. 23, 1957 INVENTOR FRANcms L CONVERSY ATTORNEYS ANCHORAGE DEVICE FOR PRE-STRESS REINFORCEMENT MElVmERS Application January 23, 1957, Serial No. 635,659

Claims priority, application France February 9, 1956 3 Claims. (Cl. 287-85) It is known that in order to anchor a bundle of stretched wires at its extremity, it has already been proposed to spread out this bundle of wires inside a frusto-conical cavity and to hold it there by means of an anchorage cone which is forced into the said cavity. In this case,

each wire is gripped between the wall of the frusto-conical cavity and the wall of the cone, along two diametrically-opposite generatrices of this wire. This anchorage is only satisfactory if all the wires are equally stretched and of exactly the same diameter, and if the cavity and the cone have a strictly conical form. The application of an anchorage of this kind is however very convenient.

In order to hold a bundle of stretched wires in a frustoconical cavity, it has also been proposed to insert between each pair of adjacent wires an anchoring key in the form of a wedge which is supported on the wall of the frusto-conical cavity. An anchorage of this type is less convenient to apply than that previously referred to, since the keys have to be placed between the wires one by one.

In order to increase the stability of this anchorage, it is known to form grooves in these keys to receive the wires. In this case also, each wire is gripped substantially along two diametrically-opposite generatrices. In this case, the fact that the cavity is not perfectly frustoconical is acceptable.

The present invention has for its object an improved anchorage device of this kind which is stable and secure, even if all the wires of the bundle are not equally stretched, and if the shape of the frusto-conical cavity is not perfect or if there are small differencesin the diameters of the wires in the wire bundle.

In the most advantageous form of embodiment of the invention, in order to maintain a bundle comprising a pre-determined number of stretched wires in a frustoconical cavity, use is made of an equal number of keys having the general shape of wedges, which are carried on the periphery'of a deformable frusto-conical body, two adjacent keys providing, at the periphery of the said deformable body, a wire housing flared towards the exterior and in which the wire can only partly pass.

In other words, the wire inserted in this flared housing and brought into contact with the opposite faces of the keys, passes beyond the external faces of the said keys.

Wires may be employed of circular or non-circular section, for example polygonal, provided that the above condition is complied with.

hired States Patent In the anchorage in accordance with the invention, each the keys themselves do not touch this wall.

In each of its sections comprised inside the anchorage device, each wire is thus subjected to three forces, two Which are almost opposite each other, namely the gripping forces of the keys, and the third which is almost exactly at right angles to the first two, which is the reaction of the wire against the wall of the frusto-conical cavity. For this reason, as this system of forces is in equilibrium, the two first forces are much greater than the third, so that each wire is essentially held by the keys which grip each wire separately.

Around the central deformable body, the keys and the wires form a conical unit, the lateral surface of which isclearly defined in magnitude but not in shape. By virtueof the flexibility of the central body, this conical assembly can adapt itself to the form of the frusto-conical cavity of the housing but, however, its penetration into this cavity is limited and clearly determined. In fact, the conical assembly is only engaged in the cavity up to a depth such that the lateral surface of the cavity engaged by the said assembly is equal to that of the assembly.

In consequence, there thus appears an additional advantage of the anchorage in accordance with the invention,

namely that this anchorage can be dismantled, which enables re-tightening operations to be carried out.

In fact, if a tractive effort is applied to the bundle of wires beyond the anchorage, a slight rearward movement of the conical assembly inside the cavity causes the contact reactions of the wires against the wall of the cavity to disappear and in consequence frees all the keys. The description Which follows below with reference to the accompanying drawings (which are given by way of example only and not in any sense by way of limitation) will make it quite clear how the invention may be carried into effect, the special features which are brought out, either in the text or in the drawings, being understood to form a part of the said invention.

Fig. 1 shows in perspective an assembly of keys rigidly fixed to a deformable central body of plastic material. Fig. 2 gives a perspective view of one of these keys.

Figs. 3 and 4 show respectively in longitudinal crosssection (taken along the line IIIIII of Fig. 4) and transverse cross-section along the line IV-IV of Fig. 3, the assembly of an anchorage in accordance with the invention.

Fig. 5 shows the transverse stresses to which a wire is subjected in a transverse section of the anchorage.

Fig. 6 shows a longitudinal cross-section of a particular construction of the anchorage cavity.

Fig. 7 shows an alternative form of construction of the keys and of their deformable support.

Fig. 8 shows an alternative form of construction of the key and the deformable body in one single piece.

Fig. 9 shows in -cross-section corresponding to the line IX-IX of Fig. 8, an anchorage obtained with a device such as that shown in Fig. 8.

In Fig. l, the keys 1, in the form of wedges, are of strong material and preferably of hard steel. They project from the external surface of a deformable frustum of a cone in which they are partly embedded. This section of cone may be for example of rubber or of any kind of plastic material. However, the deformable conical section 2 may, if so desired, be of a more rigid material but practically without great resistance to distortion, for example of cement mortar. As the mortar is crushed, a frustum of a cone of this kind also allows the small relative displacements of the keys which are necessary, as will be seen later, but it has the disadvantage that it cannot be used again.

In all cases, however, it will be apparent that the core constitutes means for yieldably supporting the keys in their conical formation for relative displacement circumferentially into clamping engagement with the wires interposed between adjoining keys.

The cone frustum 2 is preferably pierced longitudinally with a hole 3 which, when the anchorage is used as a pre-stressing member, permits of the injection of a product adapted to ensure the preservation of the bundle of wires after the latter has been stretched; in addition, the hole 3 gives the cone frustum a greater distortability.

As can be seen from Fig. 4, each of the wires 5 of the bundle to be anchored takes its position between two keys and, touching the opposite faces of these keys, it remains projecting from the surface of the frusto-conical assembly thus formed which is housed in the frustoconical cavity 8.

If a tension stress is assumed to be applied to the part 5b of the wires 5, this tension moves the wires which in turn move the keys, so that this assembly becomes wedged in the frusto-conical cavity. When this wedging action is obtained by deformation of the plastic material which constitutes the cone frustum 2, the latter no longer plays practically any part.

As in the example shown, the keys 1 are preferably in the form of Wedges, that is to say they have a rectangular section in transverse cross-section; in other words, the faces 1d of the wedge 1 which can be seen from Fig. 2 are equal trapezia, the faces 11: and 1b are rectangles, as are also the faces 10. It will be apparent that the relatively opposed faces of relatively adjoining keys constitute clamping faces, which are urged toward each other in a circumferential direction to grip the wires between them.

Fig. 5 shows the stresses which act transversely on a wire. The wedges 1, of rectangular transverse section, are not subjected to any radial stress and have therefore no tendency to be displaced radially with respect to the wires 5 between which they are gripped, even if the coeflicient of friction between the wedges 1 and the wire 5 i very small; on the other hand, the wires 5 are held between two faces of wedges which form between each other an angle of in which n is the total number of wires (in actual fact,

it would be necessary to correct this value by a term which takes account of the apex angle of the cone and of the number of wires, which term is negligible in view of the proportions generally adopted). If, in the section shown in Fig. 5, f is the force of contact of a wire 5 and a wedge 1, since the assembly is in equilibrium, this force is the same at all the contacts between the wedges 1 and the wires 5; in consequence, the force F which forces the wire 5 against the frusto-cavity 8, and which is the resultant of the two forces acting on the wire 5, is the same for all the wires. In addition, this force F is very much smaller than the forces 1. This result is only obtained if the Wedges 1 do not bear against the cavity 4.

The anchorage in accordance with the invention ofiers a very great degree of safety, since all the wires 5, the tension of which is finally transferred to the wall of the frusto-conical cavity 4, are forced against this cavity by equal forces which ensures the equilibrium of this system of forces. In'ad'dition, the wires 5 cannot inpractice be displaced one with respect to another in the event of over-tension of one or a number of the wires, since the arithmetic sum of the two forces 1 which force each wire 5 against the wedges 1, is very much greater than the force F which forces the wire 5 against the frustoconical cavity 8. Finally, the anchorage could. only fail if the coefiicient of friction between the wires 5 and the wedges 1 were less than half the angle of the wedge which is generally of the order. of twooor, three degrees; experience has shown that the angle of friction is always very much greater than this value, even if the surfaces are lubricated.

While the anchorage described above, the keys 1 of which have rectangular transverse sections, is the best, it is none the less true that keys of different shapes may be quite satisfactory. Thus, these keys may have trapezoidal transverse sections, provided that the dihedral formed by the opposite faces of the wedges remains open towards the exterior so that the support reactions of the wire on these faces of the dihedral have a resultant which is directed towards the exterior. In the limit, the faces of the keys between which a wire is placed may be parallel.

The faces 10 (see Fig. 2) of the keys 1, which are in contact with the wires 5, may be provided with striations in order to increase the coetficient of friction; the faces 1c may also be cut by a groove which serves as a housing for the wire. By reason of the magnitude of the gripping forces 1, these precautions are generally not useful. 7

The anchorage which forms the object of the present invention may be employed in a passive manner, that is to say when the male cone has been placed in position as shown in Figs. 3 and 4, a tension is applied to the part 5b of the wires 5 which results in a self-locking of the wires after a small movement of the latter and of the male portion.

The anchorage may also serve as a locking member which is applied after the bundle of wires has been stretched. There is then employed an apparatus comprising two jacks, one of which, by means of an appropriate known device, grips the extremity of the wires at their portion 5a (see Fig. 3), and enables the desired tension to be applied to the pre-stress reinforcement. The other jack then operates to drive in the male cone formed by the keys 1 embedded in the plastic mass 2, until a wedging action takes place between the keys 1 and the wires 5.

In this'case, at least one key 1 and preferably a number of keys uniformly spaced apart, may slide along the deformable body and these keys are initially left in a lower position than the others. The deformable body and all the keys having been placed in position, the wedging action is completed by driving in the keys in the lower positions. As the angle of these keys is very small, it is possible in this way to obtain a perfect wedging action. The tension jack isthen released and the tension of the bundle of wires 5 is transferred to the frusto-conical above, that is to say by initiating the wedging action by the supplementary driving of at least one key.

As the anchorage is not subjected to any damage when locked, re-tightening operations may be carried out ultimately as many times as may be desired. It is only necessary to apply to the part 5a of the wires 5 a tension' which is slightly greater than the tension of the reinforcement wires 5 at their portion 5b for the wedging action on the wires to be released, which enables a higher tension than the previous tension to be subsequently applied to'them. This special feature makes it possible to re tighten very long armature wires by means of a jack b 9.c. .or me a An ec nq'mic lterm 9 smbq is shown in Fig. 6. In thestrong casing 6 formed for exsteel.

The distortable body which couples the keys to each other. may be made in other ways than that shown in Fig. 1, provided that the said keys may be laterally displaced with respect to each other without excessive force.

In the example shown in Fig. 7, the wedges are welded or stuck on a frusto-conical metal tube 9. The wall of the tube 9 should be thin in order that its rigidity does not interfere with the relative movements of the wedges 10. The assembly shown in Fig. 7 may be replaced if so desired by a one-piece metal casting or by punching from a homogeneous material, the relative movements of the parts which form keys being facilitated by the small thickness of the sections which connect them together, which sections may be perforated.

The construction in a single piece, by casting or stamping, of an assembly such as that shown in Fig. 7, is difiicult by reason of the lack of taper of the members which form the keys and of the small thickness of the sections which join them together. the manufacture of the devices in accordance with the invention in a single piece, it is preferable to give a trapezoidal form to the rectangular sections of the wedges, the smaller side of the trapezium being located towards the exterior.

Fig. 8 shows a similar device; Fig. 9 shows the trans verse cross-section following the line IX-IX of the anchorage shown in Fig. 8. It can be seen that the faces 12a of the same key 12 are the faces of a dihedral which is open towards the interior. The result is that the thinned portions 13 of the central core are compressed. The thickness of these thin portions-13 is preferably chosen in such manner that the metal of which they are formed is subjected to an average compression stress in the vicinity of its elastic limit when the anchorage is in operation; due to this fact, these thinned portions when compressed, have only a negligible resistance to bending. The result is that at the moment of locking the anchorage, the latter can become deformed in order that the assembly constituted by the wedges and the wires bears perfectly against the frusto-conical cavity which encloses it.

The anchorage member is preferably constituted by a metal which has a high elastic limit and which can withstand large plastic deformations before fracture.

The anchorage shown in Figs. 8 and 9 is in theory less secure than the anchorages formed by means of keys, the transverse section of which is rectangular, since the forces which are produced by contact of the wires and the keys are smaller in the case of the device shown in Figs. 8 and 9 than for these previously described; in addition, this latter anchorage lends itself less well to frequent re-use, since the metal of which it is made becomes finally coldhardened; on the other hand, the slackening-01f of the anchorage shown in Figs. 8 and 9 at the moment when the tension stress of the cable is transferred to it is less than in the case of the anchorages which comprise wedges having rectangular cross-section.

The embodiments illustrated in Figs. 7, 8 and 9 have a further advantage: after the anchorage of a first layer of wires in a frusto-conical cavity, it is possible to employ the frusto-conical cavity of the anchorage member itself for the anchorage of a second internal layer of wires concentric with the first, by means of a second anchorage member of smaller diameter than the first and which is housed inside the latter. In theory, the frusto-conical cavity of this second anchorage member may be employed in its turn to form a third anchorage, and so on.

However, with the usual dimensions of the cables and Thus, in order to facilitate frusto-conical anchorage cavities, the anchorage power,

that is to say the tension stress to which the wires are subjected, capable of being obtained in one of these interior cavities, is hardly equal to half the anchorage power which can be obtained in the frusto-conical cavity having the next higher dimensions. In other words, by employing two concentric anchorage members such as shown in Figs. 8 and 9, the anchorage power is only increased by one and a half times, and with three concentric members by one and three quarter times. There is thus little advantage to be gained in arranging more than three anchorage members, one inside the other.

It will of course be understood that modifications may be made to the forms of embodiment which have been described above, in particular by the substitution of equivalent technical means, without thereby departing from the spirit or from the scope of the present invention.

What I claim is:

1. In combination with an annular member defining an inwardly converging conical cavity, an anchoring device for securing an open-centered bundle of stretched wires against inward movement through said inwardly converging conical cavity, comprising a plurality of keys disposed in relatively circumferentially spaced frusto-conical formation, a core of deformable material within said formation yieldably supporting said keys for relative radial and circumferential movement, said keys having relatively opposed wire clamping faces each facing in a circumferential direction relative to said conical formation and extending parallel to the conical axis of said formation, with each circumferentially opposed pair ,of said faces of relatively adjacent keys converging radially inwardly of the formation, said pair of faces being relatively circumferentially spaced apart at their radially outer edges a distance greater than the diameter of the wire to be clamped therebetween, said pair of faces being circumferentially spaced apart at their radially inner edges a distance less than the diameter of said wire and converging inwardly to a relative spacing equivalent to the wire diameter at a radial location spaced inwardly from said outer edges a distance less than the radius of said wire, whereby the wire is caused to project radially outwardly from its associated keys.

2; The combination of elements as defined in claim 1 wherein said radial location is spaced outwardly from said core a distance in excess of the wire radius to maintain the wires out of contact with said core.

3. In combination with an open-centered bundle of stretched wires, an annular structure defining a conical cavity through which said wires extend, and an anchoring device securing said bundle of wires against movement through said cavity in the direction of its convergence, said anchoring device comprising a plurality of keys disposed in relatively circumferentially spaced frusto-conical formation within and substantially coaxial to said cavity, a core of deformable material within said formation yieldably supporting said keys for relative radial and circumferential movement, said keys converging axially in the same direction as said cavity, and having relatively circumferentially opposed wire clamping faces each facing in a circumferential direction relative to said conical formation and extending parallel to the conical axis of said formation, each circumferentially opposed pair of said faces of relatively adjacent keys converging radially inwardly of the formation for reception therebetween of one of said wires, the relative spacing and angle of convergence between said faces of each pair being so proportioned to the cross-sectional dimensions of their associated wire as to receive the wire and support same in radially spaced relation from said core, with the wires projecting 

